Forty-five years ago on July 29, 1981, 20-year-old Princess Diana captivated hundreds of millions as she unfurled her famous 25-ft. train on the steps of St. Paul’s Cathedral to marry Britain’s most eligible bachelor, Prince Charles, 32. For many, it was the ultimate fairy tale.

A recently uncovered letter written by Diana during the couple’s honeymoon — part of an auction lot that sold for approximately $5,330 in July — seemed to reinforce that image.

“Its [sic] wonderful being married — I think its [sic] safe to say that after two months!” the new Princess of Wales wrote to a school friend. “We had a blissful honeymoon with endless sun and luckily calm seas… We are now up in Scotland until the end of October, which is a big treat for us — I adore being outside all day & hate London!”

Years later, however, Diana would describe that same honeymoon very differently — one marked by loneliness, bulimia and growing anguish over Charles’ former girlfriend, Camilla Parker Bowles, who would later become Queen Camilla.

“Diana often gave different accounts to separate people,” says royal biographer Catherine Mayer. “It’s easy to imagine why she’d write to a school friend saying everything is lovely. If you’ve bought into it yourself, you’d want to believe it for as long as possible. Even if you’re unhappy, it might feel better to let the world know that you’re blissful.”

The letter was written during the honeymoon’s final stop — an extended stay at Balmoral Castle after Diana and Charles spent 12 days aboard the royal yacht Britannia. During the Mediterranean cruise, Charles wrote to a friend: “All I can say is that marriage is very jolly, and it’s also extremely nice being together in Britannia. Diana dashes about chatting up all the sailors and the cooks in the galley etc. while I remain hermit-like on the veranda deck, sunk with pure joy into one of Laurens van der Post’s books.”

Diana too appeared every bit the happy newlywed. “She was only 20 and hadn’t been on a yacht like that before,” says Ingrid Seward, author of the upcoming Catherine & Diana. “It was all so exciting.” Asked by reporters in Balmoral about married life, Diana smiled and said she’d “highly recommend” it.

Behind that optimistic portrait, however, the honeymoon was proving far more complicated. In the days leading up to the wedding, Diana had already sensed Charles’ enduring attachment to Camilla after discovering a “goodbye” bracelet he intended to give her.

Mayer also recounts in Divide and Rule that Charles became “in a lather about the loss of a cufflink just before the pre-wedding ball at Buckingham Palace.” It was part of a set Camilla had given him — something Diana was “well aware” of.

Diana later revealed to biographer Andrew Morton, through recordings made for his 1992 biography, that she struggled with chronic bouts of bulimia during the honeymoon and was consumed by thoughts of Camilla.

“At night, I dreamt of Camilla the whole time,” she recalled.

She also recounted that, while the newlyweds compared schedules onboard the yacht, two photos of Camilla fell out of Charles’ diary. Then came another painful reminder: “Cufflinks arrive on his wrists—two ‘C’s entwined like the Chanel ‘C’s. Got it in one; knew exactly. ‘Camilla gave you those, didn’t she?’ He said, ‘Yes, so what’s wrong? They’re a present from a friend.’ And, boy, did we have a row.”

“Imagine having to move in with your in-laws on your honeymoon,” Seward says. “She’s a girl who lived on romantic novels. This was not what she hoped for.”

The cracks that surfaced during the honeymoon would only deepen. Charles and Diana, who had met just 13 times before becoming engaged, separated in 1992 and divorced four years later. Both acknowledged relationships outside their marriage, including Charles’ renewed romance with Camilla, whom he wed in 2005.

Looking back, Mayer sees the honeymoon as the first clear sign that the fairy tale had never truly existed. “The thing that is so striking about Charles and Diana is they got married understanding almost nothing of each other — or what they were doing to themselves.”